hw/i386/amd_iommu: Fix IOMMU event log encoding errors

Coverity issues several UNINIT warnings against amd_iommu.c [1]. This
patch fixes them by clearing evt before encoding. On top of it, this
patch changes the event log size to 16 bytes per IOMMU specification,
and fixes the event log entry format in amdvi_encode_event().

[1] CID 1487116/1487200/1487190/1487232/1487115/1487258

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Wei Huang <wei.huang2@amd.com>
Message-Id: <20220422055146.3312226-1-wei.huang2@amd.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>